A cell in present cellular radiotelephone communications systems typically includes six directional antennas, centrally located in the cell, each radiating into a 60.degree. sector of the cell. A plurality of these cells is combined to form a cellular radiotelephone communications system covering a geographical area. This cellular system enables mobile radiotelephone traffic to communicate with landline telephone networks and other mobile radiotelephones while moving through the area. An example of such a radiotelephone can be seen in Motorola manual #68P81049E55 available from Motorola, Inc., 1313 East Algonquin Road, Schaumburg, Ill. 60196.
Each cell has a number of frequencies, transmitted by low power transmitters, assigned to it that cannot be used in adjacent cells because of frequency interference problems. Due to the low power of the cell's transmitters, however, the same frequency can be reused in other cells, referred to as co-channel cells, in the same metropolitan area
A radiotelephone operating in a cellular system must identify itself to the system servicing the radiotelephone, allowing the system to interface the radiotelephone to the landline telephone system. The identification typically informs the system that the radiotelephone is active in that particular system in addition to telling the system the telephone number assigned to the radiotelephone. This identification is accomplished when the radiotelephone is powered up and receiving but not transmitting, in other words, in an idle state.
Autonomous registration is such an identification method. U.S. Pat. No. 4,775,999 to Williams, assigned to Motorola, Inc., describes such a registration method. This registration is performed at various time intervals to indicate that the radiotelephone is active in the system.
During a call, when a mobile radiotelephone moves from one cell to another, it must be handed-off to the next cell in order to continue communicating. This hand-off procedure is accomplished as follows for the analog cellular system. The cell serving the mobile and neighboring cell sites scans (takes samples of) the received mobile signal strength. This information is collected at the switch and a decision is made whether to hand-off the mobile to a neighboring cell. For the U.S. Digital Cellular System, as outlined in the Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) Standard IS-54 document, the mobile radiotelephone also collects channel quality information (channel bit error rate (BER) and signal strength) on its own channel and signal strength measurements on neighboring cell channels. This information is transmitted to the serving cell site as a mobile assisted hand-off (MAHO) message to be added to the base scan information for making a hand-off decision.
By reducing the size of the cells, frequency reuse can be increased, thereby increasing the amount of radiotelephone traffic a cellular system can handle. As the size of the cells are reduced, the shape becomes less circular and more elongated to provide coverage to the streets. This type of elongated cell is illustrated in FIG. 1.
When the size of the cell is reduced, however, the hand-off method must be changed. Since the cells are now smaller, the mobile radiotelephone travels between cells more frequently, requiring more frequent hand-offs. This requires the cell base station to spend an inordinate amount of time evaluating hand-off conditions, reducing the cell's traffic handling capacity. There is a resulting need for a hand-off method for use in small radiotelephone cells allowing the cells to operate at normal capacity.